ON THE PORCUPINE RIVER TUNDRA, Yukon Territory — Here on the endlessly rolling and tussocky terrain of northwest Canada, where man has hunted caribou since the Stone Age, the vast antlered herds are fast growing thin. And it's not just here.

Across the tundra 1,500 kilometers (1,000 miles) to the east, Canada's Beverly herd, numbering more than 200,000 a decade ago, can barely be found today.

Halfway around the world in Siberia, the biggest aggregation of these migratory animals, of the dun-colored herds whose sweep across the Arctic's white canvas is one of nature's matchless wonders, has shrunk by hundreds of thousands in a few short years.

From wildlife spectacle to wildlife mystery, the decline of the caribou — called reindeer in the Eurasian Arctic — has biologists searching for clues, and finding them.

They believe the insidious impact of climate change, its tipping of natural balances and disruption of feeding habits, is decimating a species that has long numbered in the millions and supported human life in Earth's most inhuman climate.

Many herds have lost more than half their number from the maximums of recent decades, a global survey finds. They "hover on the precipice of a major decline," it says.

Animals used come ‘by the hundreds’The "People of the Caribou," the native Gwich'in of the Yukon and Alaska, were among the first to sense trouble, in the late 1990s, as their Porcupine herd dwindled. From 178,000 in 1989, the herd — named for the river crossing its range — is now estimated to number 100,000.

"They used to come through by the hundreds," James Firth, 56, of the Gwich'in Renewable Resources Board said as he guided two Associated Press journalists across the tundra.

Off toward distant horizons this summer afternoon, only small groups of a dozen or fewer migrating caribou could be seen grazing southward across the spongy landscape, green with a layer of grasses, mosses and lichen over the Arctic permafrost.

"I've never seen it like this before," Firth said of the sparse numbers.

More than 50 identifiable caribou herds migrate over huge wilderness tracts in a wide band circling the top of the world. They head north in the spring to ancient calving grounds, then back south through summer and fall to winter ranges closer to northern forests.

The Porcupine herd moves over a 100,000-square-mile range, calving in the Arctic National Wildlife Refuge, near Alaska's north coast, where proposals for oil drilling have long stirred opposition from environmentalists seeking to protect the caribou.

The global survey by researchers at the University of Alberta, published in June in the peer-reviewed journal Global Change Biology, has deepened concerns about the caribou's future.

Drawing on scores of other studies, government databases, wildlife management boards and other sources, the biologists found that 34 of 43 herds being monitored worldwide are in decline. The average falloff in numbers was 57 percent from earlier maximums, they said.

Siberia's Taimyr herd has declined from 1 million in 2000 to an estimated 750,000, as reported in the 2008 "Arctic Report Card" of the U.S. National Oceanic and Atmospheric Administration.

The Taimyr is the world's largest herd, but Canada and Alaska have more caribou, and the Alberta study reported that 22 of 34 North American herds are shrinking. Data were insufficient to make a judgment on seven others.

Nature is not in balance
In an AP interview, Liv Solveig Vors, the June report's lead author, summarized what is believed behind the caribou crash: "Climate change is changing the way they're interacting with their food, directly and indirectly."

Global warming has boosted temperatures in the Arctic twice as much as elsewhere, and Canadian researchers say the natural balance is suffering:

Unusual freezing rains in autumn are locking lichen, the caribou's winter forage, under impenetrable ice sheets. This was the documented cause in the late 1990s of the near-extinction of the 50,000-strong Peary caribou subspecies on Canada's High Arctic islands.

Mosquitoes, flies and insect parasites have always tormented and weakened caribou, but warmer temperatures have aggravated this summertime problem, driving the animals on crazed, debilitating runs to escape, and keeping them from foraging and fattening up for winter.

The springtime Arctic "green-up" is occurring two weeks or more earlier. The great caribou migrations evolved over ages to catch the shrubs on the calving grounds at their freshest and most nutritious. But pregnant, migrating cows may now be arriving too late.

"Evolutionary changes tend to take place over longer time scales than the time scale of climate change at the moment," she said. Climatologists foresee northern temperatures rising several degrees more this century unless global greenhouse gas emissions are sharply reduced soon.

Caribou herds have gone through boom-and-bust cycles historically, but were never known to decline so uniformly worldwide.

‘Very dramatic trends’ seen in declineLeading Canadian specialist Don Russell, coordinator of a new global network formed to more closely monitor what's happening to the herds, said experts are focusing on "what has changed between this decline and previous declines."

"We've seen a number of areas where climate change is playing a big role, and we see some very dramatic trends," he said in an interview in Whitehorse, the Yukon territorial capital.

In neighboring Northwest Territories, the territorial government on Sept. 24 reported results of its aerial survey of the Bathurst herd: Its population has dropped to about 32,000, from 128,000 in 2006.

"The numbers are not getting better. There's no good news, no indication of recovery," J. Michael Miltenberger, the environment and natural resources minister, said by telephone from Yellowknife, the capital.

He said "there's a huge issue" with the Beverly herd, which numbered 276,000 in 1994, ranging over the Canadian tundra 1,500 kilometers (1,000 miles) due north of North Dakota.

"We've been flying north to south, east to west," Miltenberger said. "By our count, with the Beverly herd, they've all but disappeared."

Climate change is piling problem upon problem on the caribou, he said, including bogging them down in thawing permafrost and lengthening the wildfire season, burning up their food.

"The cumulative impact is bringing enormous pressure on the caribou," he said.

Implications for humans relying on caribouAnd that puts pressure on Canada's "first nations," who for at least 8,000 years have relied on the harvest of caribou meat for the winter larder, have settled along migration routes, have built their material culture around the animal — using skin, bones and sinews for clothing, shelter, tools, thread, even their drums.

Such reliance is mirrored in Siberia and northern Scandinavia, where the Sami people make a hard living herding reindeer as livestock. Freezing rains there are reported to have forced Sami to buy fodder to substitute for ice-locked forage.

Here in the timeless, silent beauty of Gwich'in country, his people may face "hard decisions," Firth acknowledged, perhaps to limit their hunt to ease the pressure. Last week, the Yukon government took a first step, restricting hunting to bulls, to spare reproducing cows.

"The future of the Gwich'in and the future of the caribou are the same," the Gwich'in often say. But even more may be at stake.

On this summer day above the Arctic Circle, binoculars found a group of caribou being stalked and circled by a hungry grizzly bear, a needy predator and another link in an intricate, interdependent natural web that may be unraveling, year by year and degree by degree, on the tundra.